Applications - Atomic Resolution

Adama Innovations atomically sharp diamond probes offer outstanding performance with atomic resolution in vacuum, air and liquid environments. Here we demonstrate atomic resolution topography and electrical contrast for a variety of inorganic surfaces in various environments.

Atomic resolution imaging represents the pinnacle of AFM performance. This technique has been invaluable in the study of a wide range of materials at the Angstrom scale. Performance at this level is only possible when using a probe which is itself atomically sharp. Adama Innovations is, for the first time, making atomic resolution easily accessible with sharp conducting diamond probes that consistently deliver atomic resolution imaging under UHV, ambient and liquid environments. The hardness and resilience of the diamond tip enables the collection of more data at higher resolution for longer.

Figure 3 demonstrates atomic resolution imaging in Milli-Q water on a mica surface using FM-AFM. Here the Adama Innovations sharp diamond probe delivers atomic resolution at both large (50 nm) and small (15 nm) scan sizes and evidence of defects in the lattice structure. In order to image with this resolution the cantilever must be driven at an amplitude smaller than a water molecule (< 0.25 nm) such that there is never a solvent molecule between the tip and the surface. The resolution demonstrated in Figure 3 is difficult to obtain in a pure water environment when using conventional silicon cantilevers. As such most researchers add salt to the solution in order to increase the force gradient near surface the surface.[1] Such modifications of the imaging environment are not necessary when using Adama Innovations sharp conducting diamond probes. With the technology to reliably fabricate super sharp tips, in ultra-hard single crystal diamond, Adama Innovations is providing a easier path to atomic resolution imaging.

Electrical characterization using Atomic Force Microscopy (AFM) is generally performed by either measuring the electrostatic force between the probe and the surface at some fixed distance (e.g. EFM, KPFM) or by measuring the current passing through the tip when it is in contact with the surface (e.g. C-AFM, SSRM). These types of measurements require conducting cantilevers which typically use a metal or other conducting coating over a silicon cantilever. These coated probes typically deliver poor spatial resolution and wear performance. Adama Innovations sharp conducting diamond cantilevers offer high resolution imaging which can be maintained for many hours.

Figure 1 shows the electrostatic characterization of folds in a graphene sheet printed onto silicon. Here we can observe higher surface potential in regions where the graphene is loosely bound to the substrate. Note that the bright feature in the upper left is apparent in both adhesion (middle) and surface potential (bottom) but is absent from the topography image (top).

Figure 2 shows the electrostatic characterization of one atom thick layers of graphene and boron nitride grown on a copper foil substrate. The undulating surface of the copper masks the presence of these thin films in the topography image (left) but they are clearly visible in the surface potential image (right).

The Spreading Surface Resistance (SSR) of 20 - 30 nm diameter silver nanowires is shown in Figure 3. Here the Adama Innovations sharp conducting diamond probe is able to resolve individual grains in the nanowires in both topography (top) and current (bottom) whilst being scanned across the surface under a constant applied load.

PeakForce-TUNA is a technique where a force curve is performed for every pixel of an image allowing for mechanical characterization of the sample and measurement of the current during the tip-sample contact. Figure 4 Shows high resolution images of individual grains in a gold film. Here the grain boundaries are visible in the modulus image (top) whilst a detailed map of the conductivity within each grain shows an average diameter of 25 nm (bottom).

Figure 5 demonstrates the lifetime advantage of Adama Innovations sharp conducing diamond probes. Th is 3.3 megapixel image was acquired over 4 hours and shows the current flow thought a network of carbon nanotubes on a silicon substrate. Here the current is flowing from the gold electrode (bottom) through the carbon nanotubes to the probe. A high resolution scan (right) show that the tip radius of 8 nm is maintained after several hours of scanning.

(1) 7.5 x 3.5 um PeakForce-KPFM scan (Bruker, Multimode 8) of graphene folds on a silicon substrate with Adama 80 N/m Super Sharp probe. A bright vertical band is visible in both the adhesion (Middle) and surface potential (Bottom) images but not topography (Top). The higher surface potential may be attributed to weaker bonding between the graphene and the substrate in this region. (Sample: Georg Duesberg, Trinity College Dublin).

Here we have demonstrated several examples of the superior resolution, lifetime and conductivity of Adama Innovations diamond probes. These probes are suitable for all electrical modes with a wide range of spring constants with tip radius options of < 10 nm (Apex Sharp) and < 5 nm (Super Sharp).

(2) 30 um KPFM scan (Asylum Research, Cypher-S) of BN (Dark Triangles) and graphene on Cu with an Adama AD-0.5-AS probe. Strong contrast can be observed in the surface potential (Right) but not the topography (Left). A graphene flake decorated by BN is evident in the surface potential image (Upper Left). (Sample: Asylum Research).

In order to map the mechanical properties of materials the AFM probe is indented into the surface and a model (e.g. Hertz, DMT, JKR) is used to fit the measured force versus indentation profile. As the technique requires one indentation per pixel traditional force mapping was slow and yielded low resolution images. New techniques including Fast Force Mapping, PeakForce QNM and AMFM-AFM allow for nanomechanical mapping of materials at traditional AFM imaging speeds and resolutions.

For piezoelectric materials the electromechanical response of a surface can be probed by scanning the surface with a conductive tip in contact mode and applying a AC electric signal. A lock-in amplifier is then used to measure the amplitude and phase of the cantilever motion due to the electromechanical response of the sample. Adama Innovations sharp conducting diamond probes provide an ideal probe for PFM providing higher resolution without tip damage.

Figure 2 shows PFM results for BFO and PPLN where the same tip was used for both samples. After 450 images of the PPLN surface over a 16 hour period the topography of the PPLN surface was unchanged. The BFO data was subsequently collected using the same probe and demonstrates well defined gain boundaries and no evidence of tip wear.